Self-aligned contact studs for semiconductor structures

ABSTRACT

A contact stud for a semiconductor structure is fabricated by providing a semiconductor substrate having an alignment structure, which includes a sidewall, and the semiconductor structure formed thereon, forming a sidewall spacer contiguous with the semiconductor structure and the sidewall of the alignment structure, depositing an insulating layer contiguous with the sidewall spacer so as to insulate the semiconductor structure, etching the sidewall spacer selectively to the sidewall of the alignment structure, the semiconductor structure and the insulating layer for forming a contact window opening for allowing access to the semiconductor structure, and backfilling the contact window opening with a conductive material so as to contact the semiconductor structure for forming the stud.

TECHNICAL FIELD

The present invention relates generally to semiconductor devices and,more particularly, to the structure and fabrication of a stud for asemiconductor device or structure thereof, used for connecting thedevice or structure to another electronic device or component.

BACKGROUND OF THE INVENTION

Semiconductor fabrication technology continues in a trend towardsincreasing circuit density and further microminiaturization ofsemiconductor structures. A semiconductor structure in this context isdefined as any region, device, component, or element thereof that can begrown, formed, diffused, implanted, deposited, etc. into or onto asemiconductor substrate. For example, the gate conductor in today's highspeed semiconductor transistor devices has diminished to a width of0.4×10⁻⁶ meters, and it is foreseen that this width will be furtherdecreased to 0.1×10⁻⁶ meters.

A stud is an electrically conductive element which contacts a structureor element of a semiconductor device and allows the device to beconnected with another semiconductor structure or electronic device. Asthe dimensions of a semiconductor structure decreases, the availablearea for forming a stud to adequately contact the structure alsodecreases. Thus, a high degree of accuracy is required to properly formand align a stud so as to contact a microminiaturized semiconductorstructure. In other words, increased microminiaturization ofsemiconductor structures leads to the problem of decreased alignmenterror tolerance when fabricating contact studs for such structures.

Moreover, minimizing contact resistance between a contact stud and asemiconductor structure is important for increasing speed and optimizingcircuit performance. In this regard, a contact stud is generallyfabricated so as to contact the top surface of a semiconductorstructure. Thus, the available area for the stud to contact thestructure is dictated by and limited to the width of the top surface ofthe structure.

Although further increasing of circuit layout density does not generallyrequire considerable decrease in the height of the structure, it doesrequire significant diminishment in the overall width of the structure.In other words, the aspect ratio (width to height ratio) of thestructure is decreased in order to increase circuit layout density.Accordingly, the available area of width on the top portion of thestructure for the stud to contact the structure is also decreased.Unfortunately, the area of contact between the stud and the structurecontrols the amount of contact resistance therebetween, such that adecrease in the area of contact leads to an undesirable increase incontact resistance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to further increasecircuit density and allow for further microminiaturization ofsemiconductor devices.

It is another object of the present invention to provide a manufacturingprocess which can accurately and properly form and align a contact studfor a semiconductor structure.

It is yet a further object of the invention to minimize contactresistance by increasing the contact region between a contact stud andsemiconductor structure.

It is another further object of the invention to increase the contactregion between a contact stud and a semiconductor structure bycontacting the stud along a sidewall of the structure.

It is still another object of the invention to provide a manufacturingprocess which self-aligns a contact stud along a sidewall of a structureon a semiconductor substrate.

In order to accomplish the above and other objects of the invention, aprocess for fabricating a stud for a semiconductor structure includesthe steps of providing a semiconductor substrate having an alignmentstructure, which includes a sidewall, and the semiconductor structureformed thereon, forming a sidewall spacer contiguous with thesemiconductor structure and the sidewall of the alignment structure,with the sidewall spacer being of substantially the same height as thealignment structure, depositing an insulating layer contiguous with thesidewall spacer so as to insulate the semiconductor structure, with theinsulating layer being of substantially the same height as the sidewallspacer, etching the sidewall spacer selectively to the sidewall of thealignment structure, the semiconductor structure and the insulatinglayer for forming a contact window opening for allowing access to thesemiconductor structure, and backfilling the contact window opening witha conductive material so as to contact the semiconductor structure forforming the stud.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, aspects and advantages will be morereadily apparent and better understood from the following detaileddescription of the invention, in which:

FIGS. 1A-1G are diagrammatic cross-sectional views showing a portion ofa substrate during various stages of processing during formation of astud for a gate conductor in accordance with the present invention; and

FIGS. 2A-2H are diagrammatic cross-sectional views showing a portion ofa substrate during various stages of processing during formation of astud for a diffusion region in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the following description refers to the structure and formationof studs for a gate conductor and a diffusion region, the inventioncontemplates forming studs to contact any structure or region as may berequired for a semiconductor device. Furthermore, although thedescription refers to the use of an oxide cap, a gate conductor and aspacer as alignment structures used for aligning the studs thereto, itshould be fully understood that the studs can be aligned to anystructure existing on a wafer.

FIGS. 1A-1G illustrate the formation of a stud for a gate conductor,using the gate conductor and an oxide cap as alignment structures towhich the stud is aligned. Advantageously, the contact region of thestud to the gate conductor is substantially along a sidewall of the gateconductor.

Referring initially to FIG. 1A, there is shown a portion of a siliconwafer 10 having a substrate 12 with diffusion regions 14,15, a gateoxide 16, a gate electrode or conductor 18 having sidewalls 24,26 andbeing comprised of appropriate metal (such as polysilicon, titaniumsilicide, or the like), an oxide cap 20 having sidewalls 21,22, and adielectric layer 23.

The diffusion regions 14,15, gate oxide 16, gate conductor 18 and oxidecap 20 are formed and patterned using conventional processes, such asdeposition, diffusion, implantation, photolithography, and etching.

The dielectric layer 23 is deposited, such as by chemical vapordeposition, so as to coat the diffusion regions 14,15, the sidewalls24,26 of the gate conductor 18, and the oxide cap 20. The dielectriclayer 23 is then etched selectively to the diffusion regions 14,15 andthe top of the oxide cap 20 such that only the dielectric which iscontiguous with the sidewalls 21,22 of the oxide cap 20 and contiguouswith the sidewalls 24,26 of the gate conductor 18 remain unetched. Thisremaining dielectric forms the sidewall spacers 28,30, as shown in FIG.1B.

Note that the dimensions of the dielectric layer 23 determines thedimensions of the sidewall spacers 28,30, and the dimensions of thesidewall spacers 28,30 determine the dimensions of the stud being formedto contact the gate conductor 18. Therefore, the required dimensions ofthe stud must be taken into account at the time of deposition of thedielectric layer 23.

In this example, for reasons set forth hereinbelow, the dielectric layer23 which forms the sidewall spacers 28,30 not only can be etchedselectively to the diffusion regions 14,15 and the oxide cap 24, but thedielectric layer 23 and resulting sidewall spacers 28,30 can also beetched selectively to the gate conductor 18. Moreover, oxide can bepolished selectively to the dielectric layer 23. Boron nitride (BN),silicon nitride, silicon boron nitride, and carbon boron nitride areexamples of dielectrics having such selectivity.

Preferably, the dielectric layer 23 is deposited to a substantiallyuniform thickness, and the etching technique used to etch the dielectriclayer 23 is directional or anisotropic. Illustratively, if asubstantially uniform layer of BN is used as the dielectric layer 23 toform the sidewall spacers 28,30, a reactive ion etching (RIE) techniqueprovides adequate anisotropy and etch selectivity to the diffusionregions 14,15 and to the oxide cap 20, using approximately the followingparameters:

10% CF₂ in O₂ or 12% CHF₃ in O₂

80 mTORR of pressure

400 watts of power

20 Gauss of magnetic field

Optionally, silicided junctions 32,33 are applied to the diffusionregions 14,15 to improve conductivity to these regions. The silicidedjunctions 32,33 are formed and patterned using known fabricationtechniques. Although the silicided junctions 32,33 are shown as beingapplied to the diffusion regions 14,15 subsequent to deposition of thedielectric layer 23, it should be understood that the silicidedjunctions 32,33 can also be applied to the diffusion regions 14,15 priorto deposition of the dielectric layer 23.

Next, as shown in FIG. 1C, an insulating layer 34, such as an oxidelayer, is deposited, such as by chemical vapor deposition, so as to becontiguous with the sidewall spacers 28,30 and so as to cover andinsulate the diffusion regions 14,15. The oxide layer 34 is thenplanarized or polished so that the top of the oxide layer 34 issubstantially coplanar with the top of the sidewall spacers 28,30, i.e.,the oxide layer 34 is polished until it is of substantially the sameheight as the sidewall spacers 28,30. It is to be understood thatpolishing is a method of planarizing materials during fabrication.

Since oxide polishes selectively to the dielectric used to form thesidewall spacers 28,30, i.e., the sidewall spacers 28,30 polish at aslower rate than oxide, the sidewall spacers 28,30 function as a "stop"during the polishing step. It should be realized that the use of oxideas the insulating layer 26 is for illustration purposes only and, assuch, other dielectrics which polish at a faster rate than thedielectric used for the sidewall spacers 28,30 can be used in lieu ofoxide. For example, a doped oxide such as phosphosilicate glass orborophosphosilicate glass can also be used.

Optionally, since the top of the oxide cap 20 is substantially coplanarwith the top of the sidewall spacers 28,30, the top of the oxide cap 20can be used as the polish stop in lieu of the sidewall spacers 28,30. Insuch an embodiment, a thin layer of material which is polish selectiveto oxide, such as silicon nitride, is deposited on top of the oxide cap20. Such a layer can be deposited before formation of the gate conductor18 and oxide cap 20, and must then be removed after the polishing step.However, it is preferable that the sidewall spacers 28,30 be used as thepolish stop since an additional layer of material need not be added toserve as the polish stop and removed after the polishing step. In otherwords, use of the sidewall spacers 28,30 as the polish stop whenpolishing the oxide layer 34 is desirable over use of the oxide cap 20because a reduction in the number of required steps is realized.

Next, a layer of photoresist is applied over the polished oxide layer34. The photoresist layer is patterned using standard photolithographictechniques of exposure and development so as to form a photoresistblockout mask 36, as shown in FIG. 1D. The photoresist blockout mask 36covers or blocks out sidewall spacer 30, but leaves sidewall spacer 28exposed. As further described hereinafter, sidewall spacer 28 is exposedfor etching for forming a gate conductor contact window. Although FIG.1D shows sidewall spacer 28 exposed, the choice as to exposing sidewallspacer 28 or sidewall spacer 30 depends on the requirements of thesemiconductor device being fabricated, as either sidewall spacer 28,30can be etched to form a gate conductor contact window. However, it isimportant that the photoresist blockout mask 36 be patterned to coverthe sidewall spacer which will not be used to form the gate conductorcontact window, so that it will remain unetched.

Moreover, note that the photoresist blockout mask 36 does not cover theoxide cap 20 or the oxide layer 34 adjacent to the sidewall spacer 28which is used to form the gate conductor contact window. Since thesidewall spacer 28 etches selectively to oxide, neither the oxide cap 20nor the oxide layer 34 adjacent to the sidewall spacer 28 need to becovered by the photoresist blockout mask 36. Thus, the sidewall spacer28 will be removed during etching and the oxide cap 20 and the oxidelayer 34 will remain unetched. Accordingly, a high degree of accuracy isnot required when forming the photoresist blockout mask 36 for etchingthe sidewall spacer 28.

The sidewall spacer 28 is then etched selectively to the oxide cap 20,the oxide layer 34 and the gate conductor 18. Again using BN as anexample of the material used for the sidewall spacer 28, a RIE processusing the parameters already set forth hereinabove will yield therequired selectivity.

As shown in FIG. 1E, the sidewall spacer 28 is etched until a residualportion 38 of the sidewall spacer 28 remains, thus forming a gateconductor contact opening or window 40 which is a void having boundariesdefined by the oxide layer 34, the sidewall 21 of the oxide cap 20, thesidewall 24 of the gate conductor 18, and the residual portion 38. Assuch, the etch selectivity properties of the sidewall spacer 28 allowsformation of the gate conductor contact window 40 to be "self-aligned"to the sidewall 24 of the gate conductor 18 and to the sidewall 21 ofthe oxide cap 20. Further, the sidewall spacer 28 functions, at least inpart, as a "sacrificial" structure in that the sidewall spacer 28 isetched and removed for forming the gate conductor contact window 40.

The residual portion 38 prevents the gate conductor 18 from shorting tothe diffusion region 14 through the silicided junction 32 when the gateconductor contact window 40 is backfilled with stud material.Illustratively, the residual portion 38 measures approximately 750Angstroms-1250 Angstroms in height, however, the residual portion 38 canbe of any height that will prevent the gate conductor 18 from shortingto the diffusion region 14.

The photoresist blockout mask 36 is then stripped using standardtechniques; and the wafer 10 is cleaned using, for example, ahydrofluoric acid cleaning process. The result is shown in FIG. 1F.

If required, before backfilling the gate conductor contact window 40with stud material, a liner (not shown) can be deposited so as to linethe gate conductor contact window 40. The liner reduces contactresistance between the stud and the gate conductor 18, and improvesadhesion of the stud to the gate conductor 18 and oxide layer 34, thussolving any problems of delamination. By way of example, the liner cancomprise titanium, titanium nitride, or other similar conductivematerial.

Next, the gate conductor contact window 40 is backfilled withappropriate stud material, for example titanium, titanium nitride,tungsten, or other appropriate metallurgy. As shown in FIG. 1G, the studmaterial is then polished to form the gate conductor contact stud 42which includes an exposed surface 44 serving as an electrical contactpoint for connecting to other electrical devices. Thus, the boundariesof the gate conductor contact stud 42 are defined by the residualportion 38, the sidewall 24 of the gate conductor 18, the sidewall 21 ofthe oxide cap 20, and the oxide layer 34; and the gate conductor contactstud 42 is "self-aligned" to the oxide cap 20 and gate conductor 18.

Accordingly, the contact region of the gate conductor contact stud 42 tothe gate conductor 18 is substantially along the entire sidewall 24 ofthe gate conductor 18, i.e., the contact region extends from the top ofthe residual portion 38 to the top of the gate conductor 18.Advantageously, contacting the gate conductor 18 along the sidewall 24rather than on its top portion, increases the contact area between thegate conductor contact stud 42 and the gate conductor 18, thusminimizing contact resistance therebetween.

FIGS. 2A-2H illustrate the use of a spacer as an alignment structure forforming a stud for a diffusion region.

Referring now to FIG. 2A, there is shown a portion of a silicon wafer 44having a substrate 46 with diffusion regions 48,49, a gate oxide 50, agate conductor 52 having sidewalls 54,56 and being comprised ofappropriate metal (such as polysilicon, titanium silicide, or the like),an oxide cap 58 having sidewalls 60,62, and a first layer 66 which iscomprised of a dielectric material.

The diffusion regions 48,49, gate oxide 50, gate conductor 52 and oxidecap 58 are formed and patterned using conventional processes ofdiffusion, deposition and photolithography. The first layer 66 is thendeposited so as to coat the diffusion regions 48,49, the sidewalls 54,56of the gate conductor 52, and the oxide cap 58. The first layer 66 isthen etched selectively to the diffusion regions 48,49 and the top ofthe oxide cap 58 such that only the material which is contiguous withthe sidewalls 60,62 of the oxide cap 58 and contiguous with thesidewalls 54,56 of the gate conductor 58 remain unetched. This remainingmaterial forms the first set of spacers 68,70, as shown in FIG. 2B.Thus, it is important that the dielectric material used to form thefirst layer 66 has etch selectivity to diffusion regions and oxide. Forexample, using nitride with a conventional directional etchingtechnique, such as a RIE technique, will yield the desired selectivity.

Optionally, the silicided junctions 64,65 are then applied to thediffusion regions 48,49 to improve conductivity of these regions. Thesilicided junctions 64,65 are formed and patterned using knownfabrication techniques.

Next, as shown in FIG. 2C, a second layer 72 is deposited, such as bychemical vapor deposition, so as to coat the silicided junctions 64,65,the first set of spacers 68,70, and the top of the oxide cap 58. Thesecond layer 72 is then etched selectively to the silicided junctions64,65, the top of the oxide cap 58 and the first set of spacers 68,70such that only the material which is contiguous with the first set ofspacers 68,70 remains unetched. Thus, the remaining material forms asecond set of spacers 74,76 which are of substantially the same heightas the first set of spacers 68,70, as shown in FIG. 2D.

It should be realized that the dimensions of the second layer 72determines the dimensions of the second set of spacers 74,76, and thedimensions of the second set of spacers 74,76 determine the dimensionsof the stud being formed to contact the diffusion region 48. Therefore,the required dimensions of the stud must be taken into account at thetime of deposition of the second layer 72.

In this example, for reasons set forth hereinbelow, the second layer 72which forms the second set of spacers 74,76 not only can be etchedselectively to the silicided junctions 64,65, the oxide cap 58 and thefirst set of spacers 68,70, but oxide can also be polished selectivelyto the second layer 72 and resulting second set of spacers 74,76. Inthis regard, it can be realized that selection of the material used toform the second layer 72 depends in part on the dielectric material usedto form the first set of spacers 68,70. For instance, if the first setof spacers 68,70 comprise nitride, then boron nitride, silicon boronnitride, carbon boron nitride, and silicon nitride have the requiredetch and polish selectivities to be used to form the second set ofspacers 74,76.

Preferably, the second layer 72 is deposited in a uniform thickness, andthe etching technique used to etch the second layer 72 is directional oranisotropic. Illustratively, if a substantially uniform layer of BN isused as the second layer 72 to form the second set of spacers 74,76,then a reactive ion etching (RIE) technique using the parameters setforth hereinabove provides the required anisotropy and etch selectivity.

Reference is now made to FIG. 2E. An insulating layer 78, such as anoxide layer, is deposited, such as by chemical vapor deposition, so asto be contiguous with the second set of spacers 74,76 and so as to coverand insulate the silicided junctions 64,65 and diffusion regions 48,49.The oxide layer 78 is then polished so that the top of the oxide layer78 is substantially coplanar with the top of the first set of spacers68,70 and second set of spacers 74,76, i.e., the oxide layer 78 ispolished until it is of substantially the same height as the secondspacers 74,76. Since oxide polishes selectively to the dielectricmaterial used to form the second set of spacers 74,76, i.e., the secondset of spacers 74,76 polish at a slower rate than oxide, the second setof spacers 74,76 function as a "stop" during the polishing step. Itshould be realized that the use of oxide as the insulating layer 78 isfor illustration purposes only and, as such, other dielectrics whichpolish at a faster rate than the dielectric used for the second set ofspacers 74,76 can be used in lieu of oxide. For example, a doped oxidesuch as phosphosilicate glass or borophosphosilicate glass can also beused.

Optionally, since the top of the oxide cap 58 and the top of the firstset of spacers 68,70 are substantially coplanar with the top of thesecond set of spacers 74,76, the top of such structures can also be usedas the polish stop in lieu of the second set of spacers 74,76. A thinlayer of material which is polish selective to oxide, such as siliconnitride, may be required to be deposited on top of the oxide cap 58and/or first set of spacers 68,70 for such structures to function as thepolish stop. The thin layer would then have to be removed subsequent tothe polishing step.

In accordance with the next step of the invention, a layer ofphotoresist is applied over the polished oxide layer 78. The photoresistlayer is patterned using standard photolithographic techniques ofexposure and development so as to form a photoresist blockout mask 80,as shown in FIG. 2F. The photoresist blockout mask 80 covers or blocksout spacer 70 and spacer 76, but leaves spacer 68 and spacer 74 exposed.As further described hereinafter, spacer 74 is exposed for etching forforming a diffusion contact window. Although FIG. 2F shows spacer 74exposed, the choice as to exposing spacer 74 or spacer 76 depends on therequirements of the semiconductor device being fabricated, as eitherspacer 74 or spacer 76 can be etched to form a diffusion contact window.However, it is important that the photoresist blockout mask 80 bepatterned to cover the spacer which will not be used to form thediffusion contact window, so that it will remain unetched.

Moreover, note that the photoresist blockout mask 80 does not cover theoxide cap 58 or the oxide layer 78 adjacent to the spacer 74 being usedto form the diffusion contact window. Since the spacer 74 etchesselectively to oxide, neither the oxide cap 58 nor the oxide layer 78adjacent to the spacer 74 need to be covered by the photoresist blockoutmask 80. Thus, the spacer 74 will be removed during etching and theoxide cap 58 and the oxide layer 78 will remain unetched. Accordingly, ahigh degree of accuracy is not required when forming the photoresistblockout mask 80 for etching the spacer 74. In this regard, since spacer74 also etches selectively to spacer 70, if desired, spacer 70 also neednot be covered by the photoresist blockout mask 80.

The spacer 74 is then etched selectively to the oxide cap 58, the oxidelayer 78 and spacer 68. Again using BN as an example of the materialused to form the spacer 74, a RIE process using the parameters alreadyset forth hereinabove will yield the required selectivity.

As shown in FIG. 2G, the spacer 74 is etched and removed, thus formingdiffusion contact opening or window 82 which is a void having boundariesdefined by oxide layer 78, spacer 68 and silicided junction 64. As such,the etch selective properties of the spacer 74 allows formation of thediffusion contact window 82 to be "self-aligned" to the spacer 68.Further, the spacer 74 functions as a "sacrificial" structure in thatthe spacer 74 is etched and removed for forming the diffusion contactwindow 82.

The spacer 68 prevents the gate conductor 52 from shorting to thediffusion region 48 through the silicided junction 64 when the diffusioncontact window 82 is backfilled with stud material.

The photoresist blockout mask 80 is then stripped using standardtechniques; and the wafer 44 is cleaned using, for example, ahydrofluoric acid cleaning process. The result is shown in FIG. 2H.

Next, the diffusion contact window 82 is backfilled with appropriatestud material, for example titanium, titanium nitride, tungsten, orother appropriate metallurgy. The stud material is then polished to formthe diffusion contact stud 84, as shown in FIG. 2H. It can be seen thatthe spacer 68 separates and insulates the gate conductor 52 so as toprevent the gate conductor 52 from shorting with the diffusion region 48via the diffusion contact stud 84 and silicided junction 64.

Thus, the boundaries of the diffusion contact stud 84 are defined by thespacer 68, the oxide layer 34 and the silicided junction 64; and thediffusion contact stud 42 is "self-aligned" to the spacer 68.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Thus, the invention is intended to encompass allsuch alternatives, modifications and variations which fall within thescope and spirit of the invention and the appended claims.

What is claimed is:
 1. A semiconductor device, comprising:a substrate; adiffusion region formed in said substrate; a gate conductor formed abovesaid substrate adjacent to said diffusion region, said gate conductorhaving a sidewall; an insulating cap formed above and in contact withsaid gate conductor; a residual insulating portion formed above and incontact with said diffusion region, and aligned with and contacting saidsidewall of said gate conductor, said residual insulating portion havinga width; and an electrically conducting stud formed above and in contactwith said residual insulating portion such that said residual insulatingportion prevents electrical contact between said electrically conductingstud and said diffusion region, said electrically conducting stud beingaligned with and electrically contacting said sidewall of said gateconductor, and said stud having an exposed top surface which provides anelectrical contact point for allowing electrical connection to said gateconductor, said exposed top surface being coplanar with said insulatingcap.
 2. A semiconductor device according to claim 1, wherein saidresidual insulating portion and said electrically conducting stud eachhave a width which is substantially less than the width of said gateconductor.
 3. A semiconductor device according to claim 1 or 2, whereinsaid electrically conducting stud and said residual insulating portionare of substantially the same width.
 4. A semiconductor device accordingto claim 1, wherein said gate conductor has a low aspect ratio.
 5. Asemiconductor device according to claim 4, wherein said electricallyconducting stud contacts a substantial portion of the sidewall of saidgate conductor for maximizing contact area between said electricallyconducting stud and said gate conductor.
 6. A semiconductor deviceaccording to claim 1, further comprising insulating means for insulatingsaid gate conductor and said electrically conducting stud, whereby theexposed top surface of said electrically conducting stud is notinsulated by said insulating means.
 7. A semiconductor device accordingto claim 6, wherein said electrically conducting stud has boundariesdefined by said gate conductor, said residual insulating portion, andsaid insulating means.